One thing to know before you diagnose anything: a yellowed ZZ leaflet is not coming back to green regardless of the cause. Fixing the underlying issue protects the rest of the plant — it won't rescue the leaflet that's already gone.
Rhizome rot from overwatering
This is the cause behind the large majority of yellow ZZ plants, and it's worth understanding why the plant hides it for so long. The rhizomes — the thick, potato-like structures just under the soil — are built to hold weeks of reserve water, which is exactly what makes them vulnerable: soil that stays wet keeps them sitting in moisture they were never designed to tolerate, and they begin to soften and rot from the inside. Because the plant is drawing on stored reserves rather than reacting to the soil directly, the leaves often keep looking normal for a while even as rot spreads — by the time yellowing shows up, it's frequently been progressing for weeks already.
The pattern that points here: yellowing starting at the lower leaflets, soil that's still visibly damp more than a week after the last watering, and a stem base that feels soft or gives slightly when squeezed rather than firm.
Fix: if the rhizomes are still mostly firm, stop watering and let the soil dry out completely before resuming on a much longer interval — every 3 to 4 weeks is normal for most homes, stretching further in winter. If you found soft or discolored sections, cut them away entirely with a clean blade back to firm tissue, let the cut surfaces air-dry for a few hours, and repot into fresh, fast-draining mix rather than the old soil.
Natural aging (not actually a problem)
ZZ plants are slow but steady growers, and as new stems mature, the plant will sometimes let an old, lowest leaflet or two yellow and drop on its own. The tell is that it's isolated and slow — one leaflet going at a time while the rest of the plant, including any new growth, looks completely healthy.
Fix: nothing needed. Trim the spent leaflet off if you'd like a tidier look, or leave it — either way it isn't a sign to change your care routine.
Underwatering
Far less common than rot on this particular plant, since the rhizomes are built to buy time, but it does happen — usually after a long stretch of neglect, or in a hot, bright spot that dries the pot out faster than expected. The giveaway that separates this from rot is texture: stems look limp, soft, or slightly wrinkled from the rhizome drawing down its own reserves, and the soil itself is bone-dry rather than damp.
Fix: water thoroughly until it runs from the drainage holes, then return to checking the soil by hand before every future watering rather than following a fixed calendar.
Cold damage or drafts
ZZ plant is comfortable at normal room temperature but doesn't tolerate cold well. A spot too close to a drafty window, an exterior door, or an air-conditioning vent can produce dark, almost water-soaked patches on the leaves that yellow and eventually turn black — a different look from the gradual, even yellowing of rot or aging, and one that tends to appear suddenly after a cold snap or a change in the plant's position.
Fix: move the plant away from the cold source. Affected leaflets won't recover, but the rest of the plant typically resumes normal growth once it's back in a stable, comfortably warm spot.
Nutrient deficiency
Least likely of the causes here, since ZZ plant needs very little feeding to begin with, but a plant that's gone a very long time — well over a year — without any fertilizer can show a gradual, even yellowing across older leaves while new growth stays green, as the plant redirects what nutrients it has toward the newest tissue.
Fix: feed with a balanced liquid fertilizer diluted to half strength during spring and summer, once or twice a season. Confirm the rhizomes are healthy first — feeding a plant that's actually dealing with rot doesn't help and adds unnecessary salts to already-stressed soil.